1. Field of the Invention
The present invention relates to a process for coating sheet substrates with thermoplastic polymer and in particular to an improved process for coating sheet substrates with a relatively thick coating of thermoplastic polymer in which the sheet substrate forms one die lip of the extrusion coating die employed in the coating process.
2. DESCRIPTION OF THE PRIOR ART
Techniques for the coating or lamination of a sheet substrate with thermoplastic polymer to form coated panels are known. For example the substrate and the thermoplastic polymer in the form of film or sheet may be brought into contact and laminated by the application of heat and pressure. Alternatively, the sheet substrate may be coated with thermoplastic polymer in a heated, and especially in a molten form. Adhesion of the thermoplastic polymer to the substrate is usually enhanced by applying pressure to the coated substrate before the thermoplastic polymer cools. Adhesives may be applied to the substrate before the substrate is coated with the thermoplastic polymer. Alternatively, adhesion-promoting compounds may be applied to the substrate or incorporated into the thermoplastic polymer.
Thermoplastic polymer-coated sheet substrate in the form of coated panels may be used in a variety of end uses. For example, polyolefin-coated wooden panels may be used as concrete form boards, as is disclosed by D. H. Dawes and J. P. van den Steen in Canadian Pat. No. 931,486 which issued Aug. 7, 1973. Coated panels may also be used in the conconstruction industry in interior and exterior applications. Techniques for coating wooden structures so as to obtain products of improved properties are disclosed in U.S. Pat. No. 3,544,669 of F. Schock which issued Dec. 1, 1970.
Coated panels having a coating of thermoplastic material of a thickness of at least 0.25 mm on a substantially inflexible substrate are disclosed in U.S. Pat. No. 4,004,774 of A. J. Houston which issued Jan. 25, 1977. In the coated panels disclosed by A. J. Houston, the coating of thermoplastic material is mechanically attached to the substrate. The substrate has a plurality of orifices in the surface thereof that is to be coated, the orifices being of a shape such that at least in part, the cross-sectional area of the orifice increases as the distance from the surface of the substrate increases. The coating of thermoplastic material is applied so that the thermoplastic material extends into the orifices at least as far as the portion of the orifices of increasing cross-sectional area.
An improved process for the coating of sheet substrates with thermoplastic polymer so as to form coated panels, especially the coated panels disclosed by A. J. Houston, has now been found.